Sandals are not on Mary Katherine Gallagher's feet, then they are on, then they are off again.

In the very beginning of the movie, we see a boy slowly getting into a pool. In the background, we can see a young Mary Katherine Gallagher taking off her sandals. Then we see a shot of her legs running. There are sandals on her feet. Then we see her jumping into the pool. Her feet are now bare.